Bava Metziah 30a:
The Beraisa lists several cases in which a person is exempt from the obligation to return a lost object. One of these cases is that of a Kohen who is exempt from returning a lost object that is in a cemetery. This exemption is learned from the word, "v'His'alamta" -- "And hide yourself from them," in the verse, "You shall not see the bull of your brother or his sheep cast off, and hide yourself from them; you must certainly return them to your brother" (Devarim 22:1).
Although the simple explanation of the verse is an injunction against hiding oneself from returning a lost object, the Beraisa derives from the unusual wording that sometimes one is supposed to hide himself, such as when he is a Kohen and the lost object is in a cemetery.
The Gemara asks why the Beraisa needs to derive the exemption in this case from the verse. Even without the verse, one would know that a Kohen is exempt from the Mitzvah of Hashavas Aveidah when the lost object is in a cemetery, because the Mitzvah of Hashavas Aveidah is only a Mitzvas Aseh, while the Kohen's obligation to avoid becoming Tamei is both a Mitzvah Aseh and a Lo Ta'aseh. A Mitzvas Aseh does not override an Aseh and a Lo Ta'aseh together.
Why does the Gemara say that the Mitzvas Aseh of Hashavas Aveidah does not override the Kohen's obligation to avoid Tum'ah? There is not even a Mitzvah of Hashavas Aveidah in this case! The Gemara earlier (26b) states that a person transgresses the Mitzvas Aseh of "Hashev Teshivem" only when he picks up the object and does not return it. If he merely ignores the object, he transgresses only the Lo Ta'aseh of "Lo Suchal l'His'alem" (Devarim 22:3). In the case of the lost object in the cemetery, the Kohen never picks up the object. Why is there a Mitzvas Aseh of Hashavas Aveidah at all in this case?
(a) The RAMBAN answers
that the Beraisa's case is one in which a Kohen already picked up the object (from a place which was not Tamei) in order to return it, and afterwards the wind blew it out of his hands into a cemetery.
(b) Alternatively, the RAMBAN answers
that although the Mitzvas Aseh of Hashavas Aveidah ("Hashev Teshivem") did not take effect (since the Kohen did not pick up the lost object), the Lo Ta'aseh of Hashavas Aveidah ("Lo Suchal l'His'alem") would have overridden the Kohen's obligation to avoid Tum'ah had that obligation been only a Lo Ta'aseh. The mitzvah of Hashavas Aveidah begins with actually picking up the lost item. Everything done before hand is only a preparation to the Mitzvah and does not override the Lo Ta’aseh of Tumah.
(c) The Ran disagrees with the Ramban
and asserts that a person who does not pick up a lost object does transgress the Mitzvas Aseh of "Hashev Teshivem." How does the Ran understand the Gemara earlier (26b) which says that when a person waits and does not pick up a lost object until he knows that the owner of the object had Ye'ush, and then he takes the object for himself, he transgresses only "Lo Suchal l'His'alem"? The Ran explains that the Gemara there refers to a person who watches the lost object until the owner has Ye'ush. Such a person does not transgress "Hashev Teshivem" since he is watching the object the entire time, ensuring that nothing happens to it. Even though he watches it for selfish reasons, he is acting in a manner which would ensure the protection of the object and its safe return to its owner. When the owner has Ye'ush, he gives up hope of retrieving his object and effectively lets whoever finds it keep it. On the other hand, when a person sees a lost object and walks away from it, he immediately transgresses "Hashev Teshivem" since he does not guard the lost object at all. The mitzvah of Hashavas Aveidah begins when one sees the lost item and continues till the lost item is returned. Accordingly, the Gemara says that if the Kohen walks away from the lost object, he transgresses the Mitzvas Aseh of "Hashev Teshivem."
